This invention relates to the field of Hot Wire Chemical Vapor Deposition (HWCVD) deposition as used to produce semiconductor thin film(s) on a substrate member.
Thin film semiconductors find utility in a variety of electronic devices, that are useful in applications such as active matrix liquid crystal displays, solar panels, and other related technologies.
Conventional HWCVD processes use a thin foil or a small diameter metal wire or filament, such as tungsten, tantalum, or molybdenum, as the hot element of the process. The process"" hot element is heated to a high temperature, and a gas such as silane is caused to flow over, or into physical contact with, the hot element. The hot element operates to break down the gas into its constituents. These constituents then migrate to the location of a substrate member whereat a semiconductor film is formed.
Breakdown of a gas such as silane produces, as a by product, a large quantity of hydrogen. It is to be noted that hydrogen dilution may also be useful in the fabrication of certain semiconductor films.
There are several issues which are an impediment to the successful commercialization of the HWCVD technology. First, hot elements such as small diameter wire filaments are normally only about 1 mm in diameter. As a result, the manufacturing longevity of these small diameter filaments is generally limited to the deposition of about a 10-20 micron thick semiconductor film, and often less.
Also, it is believed that the surface of the prior art thin metal filaments are converted into silicides (in the presence of silane), which silicides eventually penetrate the entire depth of the filament. The presence of suicides is believed to promote wire brittleness, and this brittleness effect can be exacerbated by the hydrogen that is present during the deposition process. Therefore, the small diameter filament that is used in prior HWCVD processes usually must be replaced after a very short time period of manufacturing use.
The diameter of prior hot wires or filaments used in HWCVD processes is small, for example on the order of about 1 mm, and the filaments are physically supported at the opposite ends thereof, usually so that the filaments extend in a horizontal direction. These small diameter filaments tend to expand in a linear or horizontal direction during filament heating, and during semiconductor deposition. As a result, the hot and thin filaments tend to physically sag, thereby producing a bowed or arc-shaped filament. This bowed shape of the hot filament promotes manufacturing irreproducibility in the semiconductor film as the substrate to filament distance is thus altered.
A general tutorial on the subject of amorphous semiconductors can be found in the text THE PHYSICS AND APPLICATIONS OF AMORPHOUS SEMICONDUCTORS by Arum Madan and Melvin P. Shaw, Academic Press, Inc., 1988. As discussed therein, amorphous semiconductors can be roughly divided into hydrogenated amorphous silicon (a-Si:H) type alloys, and amorphous chalcogenides, wherein the classification of amorphous semiconductors is determined by the type of chemical bonding that is primarily responsible for the cohesive energy of the material. Also as stated therein, amorphous silicon films have been prepared using numerous deposition techniques such as: Glow Discharge (GD or Plasma Enhanced Chemical Vapor Deposition (PECVD)); CVD; reactive sputtering; and reactive evaporation, wherein glow discharge of a gas can be created by using either a DC or an RF electric field, with RF discharges being operable at lower pressures than DC discharges. This text also recognized that the opto-electric properties of a GD a-Si:H film depends upon many deposition parameters, such as the pressure of the gas, the flow rate, the substrate temperature, the power dissipation in the plasma, and the excitation frequency.
Very generally, a gas phase radical is incorporated into an amorphous silicon film by way of a surface dangling bond that is created via an H-abstraction reaction. This dangling bond diffuses to a lower energy site in a microscopic valley that is within the amorphous silicon film. The gas phase free radical then adsorbs to the surface of the amorphous silicon film, preferably at a high point on the film surface. Surface diffusion then brings the adsorbed radical into the vicinity of a dangling bond, where incorporation into the amorphous silicon film then occurs.
U.S. Pat. No. 4,237,150 to Wiesmann is of general interest for its description of the use of a vacuum chamber to produce hydrogenated amorphous silicon by thermally decomposing a gas containing silane and hydrogen, such as silane, dissilane, trisilane, tetrasilane and the like. In the Wiesmann process, the chamber is pumped down to a vacuum of about 10xe2x88x926 torr. The ambient pressure before evaporation is in the low 10xe2x88x926 torr range, and it rises into the low 10xe2x88x924 range during deposition. A stream of silane gas is directed from a copper tube toward a tungsten or graphoil sheet or foil that is resistive heated to 1400-1600 degrees C. Upon hitting the hot foil, a portion of the silane gas (SiH4) dissociates into a mixture of Si, H, SiH, SiH2 and SiH3. A substrate, which may be sapphire, fused quartz or silicon, is located 1-12 inches above the heated sheet/foil, and a flux of silicon and hydrogen is deposited on the substrate. The substrate can be heated, if desired, to a temperature below 500-degrees C., and preferably to 225-325-degrees C., with 325-degrees C. being optimum. At 1600-degrees C., appreciable hydrogen is generated, which hydrogen reacts with the silicon condensing on the substrate, to thereby yield an amorphous silicon hydrogen alloy. An amorphous silicon film of 2500 angstroms was obtained in about 30 minutes.
What is needed in the art of hot wire chemical vapor deposition is a method/apparatus whose improved operation results in a commercially practical production process wherein the lifetime of the hot element, and semiconductor manufacturing reproducibility, are both greatly increased.
The present invention provides a HWCVD process wherein the process"" hot or heated element comprises a plurality of generally parallel, physically spaced, coplanar, and relatively large diameter rods that are formed of an electrically conductive and a non-metallic material that has a high melting temperature (in excess of 2000 degrees centigrade) and that is inert to the constituents such as silicon and germanium, examples being carbon, graphite (the crystalline allotropic form of carbon), electrically conductive silicon carbide (SiC), and high temperature and electrically conductive ceramic.
Advantages achieved by the invention operate to convert prior HWCVD processes to a production-compatible semiconductor deposition technology. Materials other than graphite or graphite-like materials can be employed in accordance with the invention; for example, high temperature electrically conducting ceramic.
In accordance with the present invention, a HWCVD process includes a heated element in the form of a plurality of physically spaced and relatively thick graphite or graphite-like rods. Advantages of the present invention include, but are not limited to:
a) Non-metallic rod material such as graphite, has a melting temperature that is much higher than metallic tungsten; therefore, a wider range of temperature can be used during the deposition process.
b) Because the resistivity of a non-metallic graphite rod is much higher than that of small diameter metal wire, graphite rod heating elements in according with this invention can be constructed in the form of a plurality of relatively thick rods, and not thin filaments as was used in the prior art. This thick rod construction results in a graphite rod heating assembly that is more stable and more durable than prior metal filament heating assemblies.
c) Graphite is chemically more inert than metal; hence, the hot graphite rods of the present invention do not react with silicon or hydrogen radicals as easily as do prior metal filaments. This further enhances the durability of graphite hot rods in accordance with the present invention for HWCVD applications.
d) Because hot graphite rods are chemically more stable, and have an extremely high melting temperature, hot graphite rods are less likely to contaminate high quality semiconductors, such as silicon based semiconductors.
The HWCVD technique of this invention is not limited to the use of silane gas, since a usable gas combination could include, SiF4 and H2 and/or silane or other gas combinations. These gas combinations could include, but are not limited to, dichlorosilane, germane, and methane to fabricate semiconductors such as Si:Ge, SiC, SiN, GeN. This list of gasses is meant only as a guide, and the present invention is not to be limited to these gas precursors, but could include fluorine precursors to reduce density of defect states in the resulting material. Further, with the use of dopant gasses, such as PH3 and B2H6 or the like, semiconductor doping of n-type and p-type can be accomplished with silicon as the host matrix. The technique of the present invention can also be used with liquid sources to produce materials such as GaAs, GaN, SiC and the like. Other suitable n-type and p-type dopants could be used in the fabrication of semiconductor materials, such as GaAs.
As an example, to further improve the electronic quality of a poly-crystalline silicon films, such as its grain size, the film can be heat treated via a suitable temperature profile treatment. This process step could be accomplished with suitable heating and cooling techniques, such as, but not limited to, intense light illumination, passing the film through a preheated high temperature zone, etc. Heat treatment can also be simultaneously accomplished during the HWCVD process.
With an appropriate arrangement of deposition equipment and processing procedures, the present invention can be repeated, to thereby form a number of layers, such as p-type, i-type, n-type materials, to thereby form optoelectrical devices and large area modules such as solar cells, optical sensors, and thin film transistors. A major advantage of the present invention is that the deposition rate is significantly higher when compared to most CVD and Physical Vapor Deposition (PVD) methods. Further, this invention is simpler and more practical for use in production processes.
In other embodiments of the invention, semiconductor material growth is enhanced in different ways. For instance, by superimposing an external electric field, such as RF (radio frequency) or DC, on the graphite rods, or by the application of a bias electric field to the substrate or to an intervening mesh that is located between a substrate, such as glass, and the graphite rods.
As a feature of the present invention, a generally planar substrate is physically supported a short distance away from a hot element assembly having a plurality of graphite rods that are supported in a plane that is generally parallel with the plane of the substrate. The hot element assembly comprises a plurality of mutually parallel graphite or graphite-like rods, and the hot element assembly is constructed and arranged so the center-to-center distance between the graphite rods can be varied as desired. In operation, it is preferred, but not essential, that the hot-element assembly be oscillated in a direction that is generally normal to the axis of the graphite rods, and in a manner to maintain the plane of the hot element assembly at all times parallel to the plane of the substrate. It is preferred, but not essential, that the amount of this rod oscillation movement be generally equal to the center to center spacing of the graphite rods.
The new and unusual HWCVD process in accordance with the present invention finds utility in the manufacture of thin film, multi-layer, solar cells having layers in the 0.1-10 micron thickness range, gas sensor arrays, image scanners, charge coupled devices, photovoltaic modules, thin film transistors, and in the manufacture of amorphous silicon layer(s), micro-crystalline silicon layer(s), poly-crystalline silicone layer(s), superconductor films, dielectric films, and diamond-like layer(s).
The new and unusual HWCVD process in accordance the present invention also finds utility for use in a computerized cluster tool deposition assembly wherein a plurality of individually operable vacuum deposition chambers are located generally in a circle about a centrally located vacuum isolation chamber that contains a computerized robotic arm that operates to move a substrate member between the vacuum deposition chambers, as multiple layers are deposited on the substrate member in any desired sequence. The new and unusual HWCVD process in accordance the present invention also finds utility for use in an in-line system wherein a plurality of individual vacuum deposition chambers are serially arranged in a generally linear fashion.
While this invention will be described relative to the vacuum deposition on one planar surface of a one piece and generally planar substrate member, the physical configuration of the substrate member is not to be taken as a limitation on the spirit and scope of this invention. As is well known by those having normal skill in the art, the substrate can be a stainless steel foil or a plastic substrate such as polyemide.
Without limitation thereto, the present invention provides a vacuum deposition chamber having a substrate carrier that is capable of holding a generally planar or flat substrate that ranges in size from about 10xc3x9710 cm to about 30xc3x9740 cm, with the substrate member being maintained at no more than 550 degrees C. A hot rod assembly in accordance with this invention comprises a generally planar or flat grid design, with the substrate to grid distance being variable. Automated and computer controlled relative movement, or relative oscillation, occurs between the grid and the substrate member, as the substrate to grid distance remains constant at whatever distance has been selected. A selected gas is caused to impinge upon the hot rods, and gas constituents are thereby produced in the space between the grid and the substrate member. In a preferred configuration, the substrate carrier occupies a generally horizontal plane, the substrate carrier is mounted on horizontally extending tracks, vertically above the hot rod grid which also occupies a generally horizontal plane, and the substrate carrier is moved or oscillated in its generally horizontal plane.
These and other features, advantages and objects of this invention will be apparent to those of skill in the art upon reference to the following detailed description, which description makes reference to the drawing.